Airborne particle measuring devices are instruments used to determine air quality by counting and sizing the number of airborne particles in the air. Such information may then be used to determine the air quality inside a building or in the ambient air. It also is useful in assessing the cleanliness level in controlled environments, such as a cleanrooms.
Any particle measuring device has a limit on the smallest particle size it can detect. This is because, eventually, a particle may be so small that the light scattered by the particle cannot be distinguished from background noise, caused by operation of the device itself. In practical terms, the smallest particle a handheld particle counter can detect is approximately 0.5 microns (500 nanometers).
Portable (but not handheld) particle measuring devices are available that can measure particles small as 0.3 microns. These devices, however, suffer a number of disadvantages that limit their usefulness. Among these disadvantages is their cost. One of these portable particle measuring devices can easily cost $20,000.00 or more. Also, while they are nominally portable, their large size and considerable weight makes them difficult to deploy, transport and store. Additionally, when they are deployed in a cleanroom environment, they may provide surfaces upon which airborne particles may settle and accumulate.
Fixed-installation remote particle measuring systems also have the ability to detect and measure airborne particles of less than 0.5 microns. However, these are large systems, usually deployed on a facility-wide basis to provide continuous monitoring of several locations within the facility. They are only deployed at great cost and after careful evaluation and planning.
Condensation particle counters (CPC) are capable of indirectly detecting nano-particles, in some devices, as small as 2.5 nm. A CPC works by exposing the particles to the super-saturated vapor of a solvent such as butyl alcohol. The cooled, supersaturated vapor condenses upon the particles in droplets, effectively causing a particle to increase in size from, for example, 0.01 microns to 1-2 microns, at which size the particle becomes readily detectable. In addition to being complicated to use, operate and maintain, because a CPC grows all particles to the same size, it is capable only of counting particles and not of measuring and/or reporting their size.
As mentioned above, handheld particle counters produced according to conventional engineering and manufacturing methods have a low signal-to-noise ratio that prevents them from having the resolution necessary to be able to detect airborne particles any smaller than, for example, 0.5 microns. One of the most problematic components of these devices is the sensor. Conventional sensors are incapable of minimizing background noise to the degree necessary to detect particles smaller than 0.5 microns.
Another disadvantage of conventional particle measuring devices is that they do not report the actual size of the particles detected. Instead, the pulse emitted by a detector as it detects a particle is classified within a channel or bucket representing a particular nominal size. For example, in the case of a nominal size of 3.0 microns, there may be significant numbers of particles that are larger or smaller than the nominal size of 3.0 microns. Typically, the variance of all the particles from the nominal size results in a Gaussian size distribution about the nominal size. Conventionally, in selecting which particles are to be classified within the 3.0 micron channel, the processor selects those particles at the peak of the Gaussian curve and to the right and places those in the 3.0 micron channel. All particles to the left of the peak of the curve are placed in the next lower bucket. Thus, even though the device measures the size of each and every particle, once the particles are placed in size channels, the particle size data is lost or at least hidden from the user, and the values reported for the nominal size actually fall within a size range, rather than the actual measured particle sizes. While the use of buckets or channels to classify particles according to size ranges conforms to official standards for particle measuring devices, it is insufficiently precise for many high-complexity situations.